Method for providing consistent liquid pressure output from an accumulator

ABSTRACT

A method for control of the drive means for a reciprocating piston pump delivering liquid to a spring loaded piston liquid accumulator providing high volume compliance whereby the accumulator liquid volume is controlled within narrow limits by continuous control of power to the pump drive motor. The accumulator achieves high volume compliance by arranging the kinematics of a main spring loading the accumulator piston to have a negative spring rate equal to the sum of all other positive spring rates produced by a second spring used to adjust the accumulator pressure, by the diaphragm (piston), and by a flexure support for a sensor lever. This sensor lever moves with the accumulator diaphragm to actuate an optical sensor producing an electric signal indicative of small changes in liquid volume in the accumulator. This signal in turn continuously modulates the power to the motor driving the pump so as to maintain the accumulator liquid volume close to a datum value during a large portion of the pump delivery cycle. This close control of liquid volume in a high compliance accumulator provides substantially constant pulse free pressure liquid delivery from a pulsatile pump. The second spring may be adjusted to modify this constant pressure without disturbing the balance between positive and negative spring rates. Adjustment may be manual or automatic in response to liquid temperature whereby liquid pressure is automatically increased with lower liquid temperatures to compensate for increased liquid viscosity to maintain liquid flow substantially constant through an apparatus such as flow cytometry used for particle analysis or particle sorting.

This is a division of Ser. No. 09/224,405 filed Dec. 31, 1998 which is a division of Ser. No. 08/779,505 filed Jan. 7, 1997 now U.S. Pat. No. 5,915,925 issued Jun. 29, 1999.

BACKGROUND

1. Field of Invention

This invention relates to a system for delivering pressurized liquid to an apparatus, such as a flow cytormeter, and in particular to a system having an improved liquid accumulator/pump control means for providing continuous modulation of power to a liquid pump drive means.

2. Description of Related Art

Flow cytometry apparatus has commonly used a liquid suspension of particles ensheathed in a particle-free liquid wherein this coaxial flow is passed through an analysis region and thence often to a particle sorting means. Such coaxial flow systems are shown in an article by P. J. Crossland-Taylor, Nature 171, 37 (1953) and in U.S. Pat. No. 3,826,364, which are hereby referred to and incorporated herein. Sheath liquid is usually a phosphate buffered saline solution and is usually supplied to the analysis region from a closed reservoir pressurized by air from an air pressure regulator connected to a source of air at higher pressure (note items 16, 26, and 22 of U.S. Pat. No. 3,826,364). Since particle analyzers and particle sorters often depend on consistent liquid flow velocities through the analysis region, this air pressurized sheath supply system has the following shortcomings:

1) As the sheath supply reservoir empties during operation of the flow cytometer the liquid level decreases and the loss of head causes a decrease in liquid flow rate;

2) Changes in sheath liquid temperature cause changes in sheath liquid flow rate due to changes in liquid viscosity. Changes in liquid temperature can result from changing ambient air temperature or from sheath reservoir replenishment with liquid at a different temperature.

3) Replenishment of sheath liquid is inconvenient, requiring stopping operation of the flow cytometer, de-pressurizing the reservoir, opening and refilling the reservoir, re-pressurizing the reservoir and restarting the flow cytometer;

4) The pressurized reservoir has often been a stainless steel ASME pressure vessel which is both expensive and unsuitable for visual observation of liquid level in the reservoir;

5) Air dissolves in the sheath liquid in time and can later be released as bubbles as the liquid loses pressure while flowing through filters, valves, and conduits to the analysis and sorting regions. Bubbles in these regions often prevent proper analysis or sorting functions; and

6) When pressureized air supply is not available at a flow cytometer installation, then a separate air compressor, motor, reservoir, and controls must be provided.

Attempts to use gear or centrifugal pumps to pressurize sheath liquid, usually phosphate buffered saline, have not produced practical designs. Neither pump is inherently self-priming so initial start up or restart after running out of liquid requires the operator to perform special procedures such as bleeding air from the system. If either pump is kept running when liquid flow through the flow cytometer stops, then the pump will tend to overheat and be damaged. Solutions such as an overflow/overpressure line for returning pressurized liquid back to the supply reservoir or stopping the pump add cost and complexity. Also gear and centrigugal pumps suitable for long life operating with corrosive saline are expensive.

Many of these shortcomings of gear or centrifugal pumps are avoided by diaphragm pumps, particularly those with polymer housings and with elastomer diaphragms and check valves. However, diaphragm pumps require a liquid accumulator to supply pressurized liquid during the refilling stroke of the pump. Common accumulators employ a piston loaded by a spring or a bladder loaded by compressed gas or combinations thereof (as is shown in U.S. Pat. No. 4,278,403. This patent shows an accumulator 35 which operates a pump P via a switch 43 in an on/off mode from a pressure movable partition element, piston 36. This on/off mode of pump control with its dead band between On and Off conditions results in significant changes in pressure in accumulator 35. In addition the friction from seals for piston 36 and stem 41 produce inaccuracies in the sensing of pressure in accumulator 35. Also these seals are subject to wear and leakage which limit the durability of accumulator 35.

OBJECTS AND ADVANTAGES

Accordingly several objects and advantages of my invention are:

a) to provide essentially pulse-free constant pressure liquid to an apparatus, such as a flow cytometer, unaffected by liquid level changes in the supply reservoir;

b) to employ an unpressurized supply reservoir which is easy to refill, is simple and low cost, may be raised and lowered without affecting the liquid supply pressure, does not introduce air into the sheath liquid, does not require a separate air supply and valves, may be sized large to reduce replenishment frequency, may be transparent for visual observation of liquid level, and may be replenished without stopping operation of the apparatus;

c) to provide for manual or automatic adjustment of the sheath liquid pressure as required to compensate for variations in sheath liquid temperature and thereby maintain sheath liquid flow rate substantially constant and thus maintain critical flow cytometer timing such as:

Particle transit time from laser beam to drop break-off for drop-in-air sorters,

Particle transit time from laser beam to catcher tube for a catcher tube sorter;

Particle transit time between laser beams in a multiple laser analyzer or sorter;

d) to provide a novel liquid accumulator which can accept the liquid volume delivered by one stroke of a diaphragm pump with negligble change in liquid pressure; and

e) to provide a self-priming pump in a liquid supply system where the liquid contacts only non-metal parts thus avoiding metal corrosion and contamination of the liquid.

Other objects and advantages are to provide apparatus and method for delivering pressurized liquid to an apparatus which is small, simple, low cost, reliable, durable, quiet, accurate, essentially pulse-free, and which operates with low electric power.

SUMMARY OF THE INVENTION

The present invention is directed to a pulse-free, constant pressure liquid delivery system which may be adapted for connection to an apparatus, such as a flow cytometer. Preferred embodiments of the invention provide for manual or automatic adjustment of the liquid pressure to compensate for variations in liquid supply temperature and thus maintain constant liquid flow rate through the apparatus. This invention avoids many of the problems, inconveniences, and cost associated with other liquid supply systems by use of a novel liquid accumulator design and an electric motor driven reciprocating diaphram pump controlled by a simple electronic control responsive to the liquid volume in the accumulator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood, and further advantages and uses are more readily apparent, when considered in view of the following detailed description of the exemplary embodiments, taken with the accompanying drawings, in which:

FIG. 1 is a simplified schematic diagram of the liquid supply system as connected to a flow cytometer;

FIG. 2 is a cross-sectional view of the liquid supply assembly taken along the line 2—2 of FIG. 3;

FIG. 3 is a plan view of the volume sensor;

FIG. 4 is a cross-sectional view of the manual pressure adjustment;

FIG. 5 is a diagram of the kinematic features of the invention;

FIG. 6 is an electrical diagram of the volume sensor, volume control, and pump motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and to FIG. 1 in particular, there is shown a simplified schematic diagram of liquid supply system 10 supplying flow cytometer 12 with a constant liquid flow, all constructed according to the teachings of the invention. Liquid supply system 10 includes reservoir 20 connected via conduit 34 to pump 22. thence via conduit 36 to accumulator assembly 30, thence via conduit 38 to the inlet of flow cytometer 12. Accumulator assembly 30 is part of liquid supply control means 32 for supplying a constant flow of liquid to flow cytometer 12 which includes pump 22, pump motor 24, volume sensor 28, volume control 26, and adjustment actuator 18. Liquid supply system 10 further includes adjustment control 16 connected to liquid temperature sensor 14 and adjustment actuator 18.

Referring now to FIG. 2 there is shown a cross-sectional view (taken along the line 2—2 of FIG. 3) through the middle of liquid supply assembly 32 which shows accumulator assembly 30 including diaphragm 54 clamped between diaphragm retainer 56 and accumulator body 52 by screws 58, spacers 60, main support 40, and nuts 62. Diaphragm 54 is of the constant area rolling diaphragm type such as for example are manufactured by Bellofram Corporation. Diaphragm 54 is connected via piston 88, ball 90, diaphragm screw 92, and nut 94 to sensor lever 42. Sensor lever 42 is pivotably supported by flexure pivot 44 clamped to sensor lever 42 by flexure retainer 46, two screws 48, and two nuts 50 and is clamped to main support 40 by flexure retainer 47, two screws 49, and two nuts 51. A main spring 64 is attached to sensor lever 42 by main spring support screw 66 and nut 68 and is attached to main support 40 by main spring support screw 67 and nut 69. Main spring 64 is a helical extension spring with a hook at each end for insertion in a hole in spring support screws 66 and 67. An adjustment spring 70 is attached to sensor lever 42 by adjustment spring support screw 72 and nut 74 and to adjustment actuator 18 by coupling 76 and set screw 78. Adjustment spring 70 is a helical extension spring with a hook at each end for insertion in a hole in spring support screw 72 and in coupling 76. Adjustment actuator 18 is fastened to main support 40 by two spacers 82, two screws 84 and two nuts 86. The adjustment actuator 18 may be, for instance, a stepper motor driven linear actuator such as manufactured by Haydon Switch and Instrument and sold as Model No. 26541 which has 0.001 inch motion per electrical steo. Pump 22 and pump motor 24 may be an integrated assembly such as is manufactured by KNF Neuberger as Model No. NF30KVDC or NF1.30KVDC which are rated for continuous pumping at pressures up to 15 and 85 psi respectively. Pump motor 24 and volume sensor 28 are connected electrically to volume control 26. Pump 22 with pump motor 24 may be mounted on the main support 40 or elsewhere. Liquid supply assembly 32 is preferrably oriented so air is naturally purged from accumulator assembly 30 and pump 22 when liquid flows through these components during start up.

Referring now to FIG. 3 there is shown a plan view of sensor lever 42 and volume sensor 28 which is fastened to main support 40 by screws 98 and two nuts 100. Volume sensor 28 comprises an infrared light emitting diode facing an NPN silicon phototransistor encased in a black thermoplastic housing such as for example is manufactured and sold by Honeywell as Model No. HOA0890-T51. The reduced width end of sensor lever 42 is located within a slot between the light emitting diode and the phototransistor.

Referring now to FIG. 4, there is shown a cross-sectional view of the manual pressure adjustment where the adjustment actuator 18, coupling 76, and set screw 78 have been replaced by support plate 102, and manual adjustment 104 which is threadably engaged with manual adjustment coupling 106 which has a square cross-section slidably engaged with main support 40 in a square hole to prevent rotation of manual adjustment coupling 106.

Referring now to FIG. 5 there is shown a diagram of the essential kinematic features of the invention provided to facilitate explanation of the design of an accumulator having substantially infinite volume compliance. Volume compliance is de fined as a small change in accumulator volume divided by the resulting change in accumulator pressure. A,B,C&D are dimensions from the flexure pivot 44 pivot point P to the centerlines for the forces from main spring 64, adjustment spring 70, diaphragm 54, and the light beam of volume sensor 28 respectively. H is the height above pivot point P of the contact of main spring 64 with main spring support screw 66. a is the angle defined by tan a=H/A. L is the installed length of main spring 64 between contacts with main spring support screws 66 and 67. The following terms are defined here:

K_(M) is the spring rate of main spring 64—lbs./inch

K_(A) is the spring rate of adjustment spring 70—lbs./inch

K_(D) is the spring rate of diaphragm 54—lbs./inch

T_(P) is the torsional spring rate of flexure pivot 44 as installed, in inch lbs./radian. defined as the rate of change in moment about pivot point P per radian change in angle a due to motion of sensor lever 42 about pivot point P

A_(D) is the effective area of diaphragm 54 exposed to liquid pressure—square inches

P_(s) is the liquid pressure acting on the diaphragm—psi

F_(M) is the tension force of main spring 64—lbs.

F_(A) is the tension force of adjustment spring 70—lbs. $\begin{matrix} {{Accumulator}\quad {Compliance}} \\ \left( {{cubic}\quad {{inch}/{psi}}} \right) \end{matrix} = \frac{A_{D}^{2}}{K_{D} + {K_{A}\frac{B^{2}}{{\overset{\sim}{C}}^{2}}} + \frac{T_{p}}{{\overset{\sim}{C}}^{2}} + {K_{M}\frac{A^{2}}{C^{2}}} - {F_{M}\frac{\left( {L - H} \right)}{L}\frac{H}{{\overset{\sim}{C}}^{2}}}}$

Referring now to FIG. 6 there is shown a typical volume control 26 used with volume sensor 28 and pump motor 24. Typical components are IRF520 N channel MOSFET, R₁=560 ohm and R₂=470,000 ohm.

Referring again now to FIG. 1, in operation there will be liquid flow from the supply reservoir 20 through conduit 34 to pump 22 and then through conduit 36 to accumulator assembly 30 and then through conduit 38 to flow cytometer 12. Pump 22 is driven by pump motor 24 which is controlled by volume control 26 which is responsive to volume sensor 28. Referring now to FIG. 2, there is shown that as less liquid is contained in accumulator assembly 30 diaphragm 54 and piston 88 move towards accumulator body 52. In turn ball 90, diaphragm screw 92, nut 94, and sensor lever 42 also move towards accumulator body 52. Sensor lever 42 then moves to permit more light from the light emitting diode to reach the phototransistor in volume sensor 28 which increases the phototransistor conductivity. As shown on and now referring to FIG. 6, this increases the voltage between the gate G and the source S of the MOSFET which increases the current through pump motor 24 driving pump 22. Pump 22 then increases its discharge of liquid into accumulator assembly 30 which causes diaphragm 54, piston 88, ball 90, diaphragm screw 92, nut 94, and sensor lever 42 to move away from accumulator body 52. This motion of sensor lever 42 reduces the light from the light emitting diode reaching the phototransistor in volume sensor 28 which reduces its conductivity. This decreases the voltage between the gate and the source of the MOSFET which reduces the current (and the torque) through the pump motor 24 driving pump 22. Pump 22 then slows down or stops delivering liquid to accumulator assembly 70. In this manner, balance is obtained in this closed-loop control system. This balance is obtained both statically and dynamically throughout the delivery stroke of pump 22 from bottom dead center to near top dead center. However, near the top dead center the control loop tends to become unstable and a small fraction of the stroke volume of pump 22 is delivered to accumulator assembly 30 whether needed or not needed. This fraction is typically less than 10% of the stroke volume of pump 22. After passing top dead center, pump 22 refills from reservoir 20 and returns to bottom dead center rapidly for continued control of liquid volume within accumulator assembly 30.

It is clear that accumulator assembly 30 will have small but significant volume changes during each delivery cycle of pump 22. the liquid supply assembly 32 is provided with a novel kinematic design so that supply pressure P_(s) within accumulator assembly 30 is essentially unaffected by these small liquid volume changes whereby P_(s) is fixed within less than + or −0.1% fluctuation during continued operation of pump 22. This accumulator function is produced by arranging the main spring 64 kinematically to produce a negative spring rate at diaphragm 54 which numerically equals the positive spring rate at diaphragm 54 produced by the sum of the spring rates of diaphragm 54, adjustment spring 70 and flexure pivot 44. Referring now to FIG. 5 the negative spring rate effect of main spring 64 is produced by making H large enough in relation to the other parameters that as sensor lever 42 moves to increase angle a, the fractional decrease in moment arm from main spring force to pivot P is greater than the fractional increase in force from the main spring due to its greater extension. The net effective spring rate, K_(e), at the diaphragm centerline is given by: $\begin{matrix} {K_{e} = {K_{D} + {K_{A}\frac{B^{2}}{C^{2}}} + \frac{T_{p}}{C^{2}} + {K_{M}\frac{A^{2}}{C^{2}}} - {F_{M}\frac{\left( {L - H} \right)}{L}\frac{H}{C^{2}}} - {{lb}/{{in}.}}}} & {{Eq}.\quad 1} \end{matrix}$

 where

$\begin{matrix} {T_{p} = {\frac{{Ewt}^{3}}{12\quad h}{inch}\quad {{{lbs}.}/{radian}}}} & {{Eq}.\quad 2} \end{matrix}$

  inch lbs./radian  Eq. 2

where for pivot support 44:

E=Young's modulus of elasticity—psi

w=width—inches

t=thickness—inches

h=height—inches

The volume compliance, C_(volume), of accumulator assembly 30 is:

C_(volume)

$\begin{matrix} {C_{volume} = {\frac{A_{D}^{2}}{K_{e}} - {{{in}.^{3}}/{psi}}}} & {{Eq}.\quad 3} \end{matrix}$

  in.³/psi  Eq. 3

Infinite compliance for small changes in liquid volume is obtained when K_(e)=0. By setting K_(e)=0 in Eq. 1 and solving for H we obtain: $\begin{matrix} {H = \frac{L \neq \sqrt{L^{2} - {4\quad m}}}{2}} & {{Eq}.\quad 4} \end{matrix}$

 where

$\begin{matrix} {m = {\frac{L}{F_{M}}\left( {{K_{D}C^{2}} + {K_{A}B^{2}} + T_{p} + {K_{M}A^{2}}} \right)}} & {{Ew}.\quad 5} \end{matrix}$

    Ew. 5

Eq. 4 provides two solutions for H. Each positive, real solution is valid and may be used. Where two valid solutions exist the smaller H is preferred since it results in a more compact liquid supply assembly 32.

The liquid supply pressure, P_(s), in accumulator assembly 30 is: $\begin{matrix} {P_{s} = {\frac{{F_{M}\frac{A}{C}} + {F_{A}\frac{B}{C}} + F_{D} + \frac{\left( {T_{p}x\quad {change}\quad {in}\quad a} \right)}{C}}{A_{D}} - {psi}}} & {{Eq}.\quad 6} \end{matrix}$

Normally diaphragm 54 and flexure pivot 44 are undeflected from their relaxed positions and thus F_(D) and change in a are nearly zero and may usually be neglected. Eq. 6 then becomes: $\begin{matrix} {P_{s} = {\frac{{F_{M}\frac{A}{C}} + {F_{A}\frac{B}{C}}}{A_{D}} - {psi}}} & {{Eq}.\quad 7} \end{matrix}$

F_(M) is set to give the desired minimum value of P_(s) when F_(A) is zero. H is calculated from this value of F_(M) using Eqs. 4 and 5. Then F_(A) is calculated to give the maximum value of P_(s).

F_(A) is adjusted by varying the extension of adjustment spring 70 by linear motion produced by adjustment actuator 18 which moves one end of adjustment spring 70 through coupling 76. The other end of adjustment spring 70 is supported at a location opposite Pivot P so that negligible changes in moment arm B occur with small changes in angle a. Thus there is no significant change in total spring rate K_(e) as F_(A) is varied from minimum to maximum. The spring rate of adjustment spring 70 is chosen so the desired adjustment range of P_(s) can be obtained with the available linear motion of adjustment actuator 18. For the preferred embodiment the available motion is about 0.500 inch with 0.001 inch per step of the stepping motor. For an adjustment range of 0-50% of P_(s) each step therefore produces about 0.1% change in P_(s). This provides fine control of P_(s) setting.

When used with flow cytometer 12 the liquid supply system 10 is usually operated so as to increase P_(s) as liquid temperature entering the flow cytometer 12 decreases to compensate for the effects of increased liquid viscosity and thus maintain constant both liquid flow and velocity through flow cytometer 12. Constant liquid velocity allows for fixed settings for delay time in drop-in-air and catcher tube sorters as well as the transit time for cells passing between laser beams in a cell analyzer. Liquid temperature sensor 14 provides a signal to adjustment control 16 which then sends the appropriate number of electrical step signals to adjustment actuator 18 to drive it from a home or fixed starting position to the desired compensated operating position and thus apply the required extension to adjustment spring 70 to obtain the required supply pressure P_(s). Adjustment control 16 may use an EPROM or other suitable memory device to accomplish the function of a look-up table of stepper motor steps versus fluid temperature. The adjustment control 16 has conventional electronics suitable for driving the stepper motor of the adjustment actuator 18. The adjustment control 16 may be implemented in various ways by those skilled in the art and is therefore not described in more detail here.

While the adjustment control 16 is shown as responsive to liquid temperature it is obvious that it could be responsive to any suitable operating parameter of flow cytometer 12 which can be sensed to provide either a closed loop control of that parameter or a programmed bias of P_(s) produced in response to that parameter. Such parameters may be, for example, liquid flow as sensed by the transit time for a particle to pass through two laser beams or particle velocity as sensed by the time duration of a signal produced by a particle passing through the analysis region. A liquid flow parameter may also be sensed by the pressure drop across an orifice through which the liquid flows.

When such programmed or automatic control of P_(s) is not required, P_(s) may be adjusted by the apparatus shown in FIG. 4. The manual adjustment 104 is supported by support plate 102 and is threadably engaged with manual adjustment coupling 106 which is prevented from rotating by having a square cross-section slidably engaged in a square hole in main support 40. As manual adjustment 104 is rotated, manual adjustment coupling 106 moves linearly to change the extension of adjustment spring 70. This changes its force, F_(A), which in turn changes P_(s) as set forth in Eq. 7. With a 32 thread per inch thread and a 50% change in P_(s) with a 0.500 inch motion of manual adjustment coupling 106, there is about a 3% change in P_(s) for each revolution of manual adjustment 104.

In conclusion, it can be readily understood that liquid supply system 10, constructed according to the teachings of the invention provides a simple, compact, and economical apparatus for providing pulse-free pressurized liquid having no additional dissolved air at a pressure which is independent of liquid level in the supply reservoir, wherein this pressure may be adjusted manually or automatically to compensate for liquid temperature changes to provide for constant liquid flow and constant velocity of particles passing through a flow cytometer analysis and/or sorting region(s).

While the above description contains many specifications, these should not be construed as limitations on the scope of the invention, but rather as an example of one preferred embodiment of the invention. Many other variations are possible without departing from the teachings of the invention, of which a few alternatives will now be described:

The diaphragm pump 22 could be replaced by a peristaltic tubing pump or any other pump with suitable characteristics. The volume sensor 28 could be replaced with any non-contact proximity sensor such as for instance eddy current or capacitive devices. The accumulator 30 could use an unconvoluted or flat diaphragm. The adjustment actuator 18 could be replaced by any suitable electromechanical device such as for instance a rotary stepper motor driving a pinion gear coupled to a gear rack. It is also felt that adjustment actuator 18, liquid temperature sensor 14, and adjustment control 16 could be replaced by a non-electric means for adjustment of liquid pressure such as for instance a liquid thermal expansion apparatus. A sealed stainless steel bellows containing a liquid possessing a high thermal volume expansion characteristic could be placed in and exposed to the liquid passing through the accumulator body. One end of the bellows would be disposed in contact with the accumulator body. The other end of the bellows would be disposed so as to contact a compression spring interposed between the bellows and the accumulator diaphragm. In operation, as liquid temperature increases the liquid in the bellows expands, the bellows extends, the spring is further compressed, and the increased force on the diaphragm produces a decreased regulated liquid pressure P_(s). Flexure pivot 44 may be replaced with any suitable low frivtion bearing such as a ball bearing. A KNF Neuberger NF30KVDC pump which is rated for 15 psi is selected for the pump 22 and pump motor 24 combination in the preferred embodiment. For higher pressures a KNF Neuberger NF1.30KVDC pump which is rated for 85 psi continuous operation may be substituted. Both pumps are manufactured by KNF, Neuberger, Inc. of Trenton, N.J. Higher regulated liquid pressures may be obtained by the use of a smaller area diaphragm or by higher force main and adjustment springs in accumulator 30 without increasing the size of the liquid supply assembly 32. 

I claim:
 1. A method for providing a consistent liquid pressure output from an accumulator means storing said liquid by continuously modulating power to a pump drive means driving a pump in liquid communication with said accumulator means comprising the steps: a. sensing the liquid volume in said accumulator means, and b. modulating power input continuously to said pump drive means by providing increased power to said pump drive means when said liquid volume decreases and decreased power to said pump drive means when said liquid volume increases, whereby the said liquid volume in said accumulator means is maintained substantially constant.
 2. The method of claim 1 for providing consistent liquid pressure output from a liquid accumulator means by continuously modulating power to a pump drive means driving a pump in fluid communication with said accumulator means, wherein the step of modulating power input to said pump drive means by providing increased power to said pump drive means when said liquid volume decreases and creases and decreased power to said pump drive means when said liquid volume increases, further includes the step of using in the accumulator means a negative spring rate device to balance all positive spring rate devices, said negative spring rate device including an extension spring coupled to a pivotably supported lever at a point so located that a fractional change in spring force produced by a small motion about said pivot is less than the fractional change in distance from the line of action of the force of said spring to said pivot, whereby the force exerted by said lever on a diaphragm in said liquid accumulator means is substantially constant during small displacements of said diaphragm produced by liquid volume changes in said liquid accumulator means thereby maintaining substantially constant liquid pressure in said liquid accumulator with small liquid volume changes therein. 